ABSTRACT
Reactive Air Brazing (RAB) enables the joining of vacuum-sensitive oxide ceramics, such as Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF), to metals in a one-step process. However, damage may form in ceramic or joint during RAB. In this work, experimental microstructure characterization, measurement, and prediction of local material properties using finite element analysis were combined to enlighten these damage mechanisms, which are currently not well understood. Micromechanical simulations were performed using representative volume elements. Cooling simulations indicate that small-sized CuO precipitations are most likely to cause crack initiation in BSCF during cooling. The ball-on-three-balls experiment with porous BSCF samples was analyzed numerically to determine the values of temperature-dependent BSCF fracture stresses. The inversely calibrated fracture stresses in the bulk BSCF phase are underestimated, and true values should be quite high, according to an extreme value analysis of pore diameters.
ABSTRACT
The separation of oxygen from air by means of inorganic ceramic membranes requires gas-tight ceramic-metal joints that enable reliable permeation operation in the oxygen partial pressure gradient at 850 °C. Reactive air brazing is a promising method to solve this challenge. However, reactive air brazed BSCF membranes suffer from a significant strength degradation that is caused by unhindered diffusion from the metal component during aging. In this study, we investigated how diffusion layers applied on the austenitic steel AISI 314 influence the bending strength of BSCF-Ag3CuO-AISI314 joints after aging. Three different approaches were compared as diffusion barriers: (1) aluminizing via pack cementation, (2) spray coating with NiCoCrAlReY, and (3) spray coating with NiCoCrAlReY and an additional 7YSZ top layer. Coated steel components were brazed to bending bars and aged for 1000 h at 850 °C in air prior to four-point bending and subsequent macroscopic as well microscopic analyses. In particular, coating with NiCoCrAlReY showed low-defect microstructures. The characteristic joint strength was raised from 17 MPa to 35 MPa after 1000 h aging at 850 °C. In addition, the dominant delamination fracture between the steel and the mixed oxide layer, observed in the reference series with uncoated steel, could be replaced by mixed and ceramic fractures of higher strength. The effect of residual joint stresses on the crack formation and path is analyzed and discussed. Chromium poisoning could no longer be detected in the BSCF, and interdiffusion through the braze was effectively reduced. Since the strength degradation of reactive air brazed joints is mainly caused by the metallic joining partner, the findings on the effect of the diffusion barriers in BSCF joints might be transferred to numerous other joining systems.
ABSTRACT
The step from the testing of oxygen transport membranes on a lab scale to long-term operation on a large scale is a challenge. In a previous study, membrane failure was observed at defined positions of one end of the cooled tubular Ba0.5Sr0.5Co0.8Fe0.2O3-δ membranes after an emergency shutdown. To understand the failure mechanisms, strength degradation and transient stress distribution were investigated by brittle-ring tests and finite element simulations, respectively. A 15% decrease in the characteristic strength of 162 MPa was proven after aging at 850 °C and was attributed to grain coarsening. The reduction in characteristic strength after thermal shock ranged from 5 to 90% depending on the cooling rates, and from 5 to 40% after the first and 20th soft thermal cycling. Simulations indicated the chemical strains induced by a 10-bar feed air and 50 mbar permeate pressure, which caused tensile stresses of up to 70 MPa at the outer surface. These stresses relaxed to 43 MPa by creep within a 1000 h operation. A remaining local stress maximum seemed to be responsible for the fracture. It evolved near the experimentally observed fracture position during a 1000 h permeation and exceeded the temperature and time-dependent strength. The maximum stress was formed by a chemical strain at temperatures above 500 °C but effective creep relaxation needed temperatures above 750 °C.
ABSTRACT
Increasing evidence suggests positive associations between physical activity, social support, and quality of life in later life. However, the role of physical activity and social support in the influence on quality of life is unclear. In this viewpoint, definitions of central terms and key research findings on factors influencing quality of life in older adults are provided. We highlight the importance of both a physically active lifestyle and high social support in positively influencing quality of life. However, we question the direct effect of physical activity on quality of life and present data of a cross-sectional study to support the assumption that the effect of physical activity on quality of life might be influenced by perceived social support. In this cross-sectional study, self-reported physical activity, social support, and quality of life were assessed in a nonclinical sample of 100 adults aged between 60 and 80 years. Statistical analyses included correlation analyses and covariate-controlled mediation analyses. The results confirmed the research findings on positive correlations between all domains of quality of life and both physical activity and social support. Mediation analyses indicated that the effect of physical activity on all domains of quality of life was at least partially mediated by social support, when controlled for covariates. Based on the findings of published evidence and this cross-sectional study, a socially and physically active lifestyle is recommended for older adults to positively influence quality of life. However, the well-documented effect of physical activity on quality of might be an at least partially mediated effect by perceived social support. Practical and research-related implications of a potential indirect effect are discussed. Further prospective research is warranted to clarify the relationship between the variables.
Subject(s)
Exercise , Quality of Life , Social Support , Aged , Aged, 80 and over , Cross-Sectional Studies , Female , Humans , Linear Models , Male , Middle Aged , Multivariate AnalysisABSTRACT
BACKGROUND: Ski helmet use has steadily increased worldwide over the last 10 years in part as a result of preventive helmet campaigns but also in part as a result of increased media coverage after fatal injuries involving celebrities. However, a commonly reported reason for nonuse is impaired vision. OBJECTIVE: The aim of this pilot study was to investigate whether ski helmet use affects reaction time to peripheral stimuli. METHODS: A randomized controlled trial using the Compensatory-Tracking-Test (CTT) was conducted in a laboratory situation. This test measures reaction time to peripheral stimuli during a tracking task and was carried out by 10 males and 10 females (age: 22.1 ± 2.5 years) during 4 conditions in a randomized order: (A) with a ski cap; (B) with a ski helmet; (C) with a ski cap and ski goggles; and (D) with a ski helmet and ski goggles. RESULTS: Friedman-tests revealed significant differences in reaction times (ms) between the 4 conditions (p=.031). The lowest mean reaction time (± standard error) was measured for cap only use (477.3 ± 16.6), which was not different than helmet-only use (478.5 ± 19.1, p=0.911). However, reaction time was significantly longer for cap + goggles use (514.1 ± 20.8, p=0.005) and for helmet + goggles use (497.6 ± 17.3, p=0.017) when compared to cap-only use. CONCLUSION: Our results showed that ski helmet use did not increase reaction time to peripheral stimuli. This information should be implemented in future preventive campaigns to increase helmet use in skiers and snowboarders.
